1. Field of the Invention
The present invention relates to an electric system of an electric vehicle using a main secondary battery as a power source.
2. Description of Related Art
FIG. 1 shows a drive system of a conventional electric vehicle which uses a main secondary battery (hereinafter, referred to simply as a secondary battery) as the power source. This figure shows a drive system for driving two wheels by a single AC motor. In FIG. 1, the reference numeral 1 designates a secondary battery, the reference numeral 2 denotes a main contactor, the reference numeral 3 designates a fuse, the reference numeral 4 denotes an inverter, the reference numeral 5 designates an AC motor, the reference numeral 6 denotes a triple pole contactor for opening a motor circuit, the reference numeral 7 designates a reduction gear unit, and the reference numeral 9 denotes a differential gear unit.
In this drive system, the rotation of the AC motor 5 is decelerated by the reduction gear unit 7, and the decelerated rotation is transferred to the wheels 81 and 82 via the differential gear unit 9.
The inverter 4 is connected to the motor 5 through lines 60a, the triple pole contactor 6, and lines 60b, so that the triple pole contactor 6 breaks the lines 60a and 60b if a fault occurs on the side of the motor 5.
In the drive system shown in FIG. 1, the inverter 4 inverts the DC power of the secondary battery 1 into AC power, thereby controlling both the torque and rotating speed of the AC motor 5.
During the power running of the electric vehicle, the power is supplied from the secondary battery 1 to the motor 5 via the inverter 4 which inverts the DC power into the AC power, so that the wheels 81 and 82 are rotated and consequently the vehicle body is driven. During the regenerative braking, the kinetic energy of the vehicle body is regenerated to the secondary battery 1 via the wheels 81, 82, the motor 5, and the inverter 4 which converts AC to DC in the direction opposite to that during the power running.
In general, the inverter of the electric vehicle is arranged in the form of a 3-phase transistor inverter as shown in FIG. 2.
In FIG. 2, the reference numeral 401 designates a transistor, and the reference numeral 402 denotes a diode connected in antiparallel to the transistor 401. The major circuit of the 3-phase inverter is composed of six arms, each of which consists of a switching element combining the transistor 401 and the diodes 402. The reference numeral 403 designates a capacitor for smoothing the current from the secondary battery 1.
FIGS. 4A-5D illustrate voltage waveforms and current waveforms of respective portions of the drive system during the drive operation of the electric vehicle. Generally speaking, the inverter for driving the electric vehicle employs the same PWM control as industrial AC motor driving systems. These figures illustrate the PWM control method. It should be noted that FIG. 3 shows a circuit arrangement for explaining the voltages and currents in FIGS. 4A-4D (the power running operation mode), and FIGS. 5A-5D (the regenerative braking operation mode).
As is apparent from FIGS. 4A-5D, the voltage VB of the secondary battery 1 is subjected to the PWM control by the inverter 4 to be converted to the an AC-side voltage V.sub.M of the inverter 4, which takes such a waveform as shown in FIG. 4A. The waveforms during the power running operation and regenerative braking operation are similar. A waveform indicated by a dotted line in FIG. 4A represents a fundamental wave of the PWM control. The PWM control is carried out in such a manner that this fundamental waveform becomes a sine wave. Thus, the AC-side current i.sub.M of the inverter 4 takes such a waveform as superimposing higher harmonic currents on the fundamental wave current.
The examples shown in FIGS. 4A-5D are when the power factor of the fundamental waves is 1. As shown in these figures, the phases of the currents during the braking operation mode (FIGS. 5A-5D) and the power running operation mode (FIGS. 4A-4D) are opposite, and hence, the braking operation becomes the regenerative operation. Similarly, the polarity of the DC-side current i.sub.B is also inverted during the braking operation.
Since the stored energy of the secondary battery on an electric vehicle is finite as the power source, the battery must be properly recharged. This battery charging operation is a major problem in using electric vehicles.
In other words, the charging operation of the secondary battery and a charging device are essential in using electric vehicles.
FIG. 6 shows a conventional charging system. The reference numeral 100 designates an electric vehicle, in which the same reference numerals are used as in FIG. 1.
In FIG. 6, the reference numeral 300 denotes a charging device connected to the secondary battery 1 via a charging cable 400 and a charging connector 200 which is connected to the secondary battery 1. The charging device 300 is connected to a connector 220 of an external power distribution system 240 via a cable 700.
During the charging operation, the main switch 2 of the electric vehicle 100 is opened, so that the secondary battery 1 is charged with the power from the power distribution system 240 using the charging device 300.
FIG. 7 shows the conventional charging device 300. In this figure, the reference numeral 301 designates an AC-side contactor, the reference numeral 302 denotes a step-down transformer provided as necessary, the reference numeral 303 designates a diode rectifier for converting an AC voltage into a DC voltage, the reference numeral 304 denotes a chopper controlling a charging current, the reference numeral 305 denotes a reactor for smoothing a charging current, and the reference numeral 306 designates a fuse.
Generally, the charging device must charge the secondary battery 1 quickly. Therefore, the capacity of the charging device 300 must be selected to be equal to or greater than that of the inverter for driving the AC motor of the electric vehicle. Thus, the charging device 300 must be necessarily made bulky, and a charging operation becomes complicated, because the charging device must have a large capacity, and include the power converter such as the diode rectifier 300, as well.
It is essential for driving electric vehicles regardless of time and space to install charging stations as shown in FIG. 8 at many places as is common with gasoline stations for internal combustion engines.
However, it is difficult to install the above-explained conventional charging system at a large number of places, because the charging device 300 is bulky and expensive. This presents a problem in widely spreading the electric vehicles.
Furthermore, the charging device 300 has the rectifier load which is connected to the power distribution system 240. This presents another problem in that it will deteriorate the quality of power by causing higher harmonic waves in the power distribution system 240, and reduce the power factor.